Automatic selective fading control circuits



sept. 15, 1936. H MAYER `2,054,657

AUTOMATIC sELEcTIvE TADING CONTROL CIRCUITS Filed Feb. 281935 ATTORNEY Patented Sept. l5, 1936 UNITED STATES PATENT OFFICE AUTOMATIC SELECTIVE FADING CONTROL CIRCUITS poration of Germany Application February 28, 1935, Seal No. 8,597 In Germany March 28, 1934 Claims.

The present invention relates to automatic signal transmission control systems, and more particularly to circuits for compensating for the effects of selective fading.

-5 For the purpose of insuring an automatic regulation of transmission measure it is known to transmit a distinct control, or pilot, frequency, and to use theamplitude variations thereof at the receiving end for the purpose of controlling the regulator means. In transmission systems predicated upon carrier frequency, it has also been suggested in the prior art to use the carrier frequency itself for the causation of the regulator action so that the transmission of a special control, or piloting, frequency could be dispensed with. However, in arrangements of this kind, there is only one criterion for such changes in attenuation as occur over the transmission path, namely, the change in damping of one frequency, While it is known in the art that such changes in attenuation are not uniform for all of the different frequencies comprised inside the used frequency band or spectrum.

In radio transmission systems the dependence on the frequency of the changes of damping are particularly marked so that, in spite of the abovedescribed regulating methods, the quality of transmission will be impairedby selective fading.

However, itis not only in radio transmission systems, but also in the case of transmission over longY lines, where the various frequencies experience non-uniform damping. `In order that the regulator means may, to a certain degree, be adapted to these working conditions, recourse has been had in the past to frequency-dependent regulator means. .In these, as will be seen, if the regulation is insured by acting upon a -receiving amplifier, the gain is not brought about by par- 40 alle] shifting of the amplifier curves, but the gain is altered to different degrees for the different frequencies. l

These arrangements have the disadvantage that a definite frequency dependence for the regulator action is fixed' from the outset so that a definite shape of the amplifier characteristic pertains to a definite incoming amplitude of the control frequency. The changes in attenuation of transmission systems, however, do not always occur with the same frequency dependence. On the contrary, they are a' function of atmospheric influences; of the temperature; and, in the case of wire transmission systems, of the particular properties of the different lines connected together to form a Comunication path, Hence,

an adequate equalization, or compensation, of the changes in the transmission measure is impossible if the frequency dependence of the regulator action has been pre-fixed.

In another method known in the prior art, for 5 the purpose of compensating selective variations of the transmission measure, the frequency range, or spectrum, to be regulated is divided into a. number of partial bands which are separately regulated. This may be accomplished, for in- 10 stance, by that a distinct control frequency is allotted to each such partial band. If these latter are chosen sufficiently closely together, it is feasible by the aid of this method to so make the regulation of the transmission measure that the 15 frequency function of the attenuation changes will be considered. However, it will be understood that the expenditure for circuitelements attendant upon this last-named method is extraordinary. Both at the sending, as well as at 20 the receiving, end filters must be provided designed to divide the frequency spectrum to be transmitted into the partial bands. In addition, as many control frequencies must be transmitted as there are partial bands; and each of these control frequencies must be filtered out of each one of these partial bands by additional filter means, then amplified, rectified, and then used for the regulation of amplifiers co-ordinated separately to each one of the various partial bands. 30

Hence, amplifiers must also be provided for each of the partial bands, or channels. It will thus be seen that this method may be used only if, and when, the particular nature of a given practical case warrants, or makes, apparently acceptable such considerable equipment in an economic Vregard.

Now, the present invention discloses ways and means whereby a compensation of selective changes in the transmission measure which measures up to practical requirements is realizable without having the spectrum of frequencies to be transmitted divided into partial bands. According to the invention, the frequency function of ,45 attenuation of a regulator device jointly traversed by one or more of all signal frequencies of the frequency band to be transmitted is controlled inv dependence upon at least two different control, or piloting, frequencies. Because of the fact that l the regulation action is made a function of at least two different frequencies it is feasible to take into consideration in the regulating action at the receiving end the actual frequency dependence of 3.19 attenuation changes occasioned in the transl mission system. As to the rest the regulation according to the invention is preferably effected in the following fashion: Two control frequencies are sent out which are contained within the lower and the upper boundary zone in the band to be transmitted, or,else-,directlybelow or above the same. At the receiving end the amplitudepof one of these control frequencies is used fo-r regulating the absolute amount of the amplification factor.;

or gain of the receiving amplifier s o that, under the influence of these control frequencies a; parallel shift of the amplifier curve is caused.

The frequency dependence ofthe regulating action is aected by the di'ii'erencefof thezamplitudes of both control frequencies. inasmuch as the two control frequencies are located at different places in the frequencybandor spectrum tobe transmitted they will be affected to dissin'1ilar` .de-

grees upon the arising of selectivev attenuation changes with the result that the difference in their amplitudes represents a.. criterion insofaras vit indicates in what. frequency zone withink .the Vband to be transmitted therehas occurred a more or less marked dam'pingldecreasey ordamping increase. ,.Hence, according kto whether theA amplitude-,difference of the 4two control frequencies eX- ceeds or falls below. a. deflnitelmean value, the gain for the .low or. for, the highpitches or frequencies may beinfluenced in the desired manner. The control of the gain independenceupon vthe amplitude difference, as'will be noted-results in a rotation ofthe characteristic rather` than a parallelshift, yin 4.the .amplification characteristic.

= For the control of the absolute magnitude of the gain,.according to. this invention, also the'sum `total oftwo--onmore control frequencies may be utilized. lIt' is also possible .to insure the frequency-dependentregulationzof vthe gain in an- .othermanner thanfhereinbefore described. For

instance, a piloting frequency included inside the lower regionor below ,the frequency band to be transmitted maybeautilizedV for influencingl Ithe :transmission measure .for'the lower` frequencies, ,andy anotherk control frequency. located inside the :'upper, boundary regionfor above the same'may be 1 employed for the control fofthe higher frequencies.

Fundamentallyspeaking, such a control or regfulator Yactioncould sbe brought: about by the use of a controllableattenuation network -comprisning complex Aresistances which would be connected';ahead` of (and in series-with) the'receiving apparatus or the receiver amplifier. Action upon #the network by the lower control `frequency would havextobebrought'aboutin such` a way Vthat essentially only the attenuation ofgthe network for the lower frequencies is acted upon, whereas the highenoneof l,the two control frequencies would :have to influencethe attenuation for the higher frequencies,v without' `causing incidentally analteration in the damping for'the lower frequencies. Hence-,itis shown that Vthe basic idea ofthe in- ;vention maybecarriedfinto practice in various rways. r'What remains essentialin this connection ;;-is.that forthe control two or more frequencies are ters. ;-Hence, the invention providesa-solution to i; means forlthe ,division of fthe spectrum into a plu- .rality .of ,channelszfurnished with distinct control frequencies that has so far been necessary in the method hereinbefore described.

The underlying idea of the invention is useful both for wire as well as for radio transmission systems; and its advantages will be apparent both in multiple as -well as in simple transmission systems.

So far as the regulation of the gain in the receiving amplifiers is concerned, there are especially suited voltage dividers which consist of linear resistances and controllable regulating resistances. For the latter it is preferable to use xresistances'whose resistivity changes under the faction of acontrol quantity, say, a control or :.-piloting'lvoltage having such great inertia that `theylwill be able to follow and respond to slow control impulses, while operating as or like linear resistances so far as the signal currents to be sent are concerned. What may be utilized for --this purpose are the so-called hot conductors in the form ofuranium dioxide filaments in vacuo.

YInthe case of long wirelines which comprise a plurality Vof .amplifiers,the invention provides for regulation' by stages orv in sections in this manner that some of the. amplifiers are regulated by the aid of a control frequency or the aggregate voltage of two ormore frequencies by parallel shifting of the amplifier curve, and that frequency-dependent gain regulation. in dependence upon two or all control frequencies lis effected only at greater intervals. I

' Theiigures described in what follows serve to explain more fully the basic .idea of the invention withV a View to afford a better understanding of an exemplified embodiment.

In the'drawingz- Fig. 1 graphically shows the position of--the control vfrequencies in the transmission band,

Fig; 2 schematically illustrates an. embodiment of the invention,

Fig113 shows circuit details of the embodiment of Fig. 2.

Fig. 1 shows the position of the two control frequencies ,f1 and f2 in reference to the frequency band to be transmitted which extendsfrom f to f: The distance of the two control frequencies from the boundaries of the band to vbe transmitted is so chosen vthat these are readily separable or lterable by the aid of filter circuits known in the prior art.

In Fig. 2 an ,exemplified embodiment is shown schematically. In the rear of (above) the receiving amplifier V are branched 01T the two control frequencies by way of filters D1 and D2,.and these, after rectification in rectiiiersGr and G2, are then used to act upon the regulator devices A and B connected below the amplifier. The

"regulating means B which serves for causing a.

parallel shift of the amplifier curves is only controlled by the fall of potential across the resist" ance R1. The gain adjusted by the regulator device B, as will be seen, is only dependent upon the incoming amplitude of `the control frequency branchedl oif by way of the filter D1.

"The regulator device A which brings about aif frequency-dependent control of the amplification factor or'gain is fed with the difference of the voltages produced across the resistances R1 and R2 acting as a control voltage. The frequencydependence of the regulating action, as will thus be noted, in thedesired manner, is madel a function of the difference in attenuation for both control frequencies.

I'ig. 3 shows the two attenuator devices A and Bin more detail. What isV involved arevoltagef dividers which consist of the linear input resistances RA and RB and the controllable nonlinear input resistances 2 and I, respectively. The nonlinear resistances are electronic tubes whose grid biasing voltages are controlled as a function of the output potentials of the rectifiers. Instead of electron tubes recourse could be had also to other controllable resistances, say, to the hot conductors above mentioned. The resistance Rs is an ohmic resistance so that by the control of the non-linear resistance I only a parallel shift of the amplifier characteristic is produced.

The resistance arrangement RA contains one, or more, frequency-dependent resistances which conjointly with the capacity C and the non-linear resistance 2 result in the desired turn in the ampliier characteristic.

While I have indicated and described several systems for carrying my invention into effect, it Will be apparent to one skilled in thc art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be made Without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:

1. In combination with van amplifier of high frequency energy, the latter comprising a band of frequencies and at least two spaced control frequencies located adjacent the upper and lower limits of the band, an energy input circuit for the amplifier, an output circuit for the amplifier, means coupled to the output circuit for deriving a direct current voltage from energy of one of said control frequencies, a second means coupled to the output circuit for deriving a second direct current voltage from energy of the other control frequency, means in the input circuit, responsive to one of said voltages, for controlling the amplitude of said band frequencies in a substantially uniform manner, and a second means in said input circuit, responsive to the differential of both voltages, for controlling the amplitude of said band frequencies in a non-uniform manner.

2. In a system as defined in claim 1, one of said control frequencies being disposed below the lower limit of the frequency band, and the other control frequency being located above the upper limit of the band.

3. In a system as defined in claim 1, each of said output circuit means comprising a control frequency filter followed by a rectifier.

4. In a system as defined in claim 1, each of the input circuit means comprising an electron discharge device having an electrode connected t its corresponding direct current voltage deriving means.

5. A method of regulating the signal transmission through an amplifier system which consists in impressing on said amplifier energy comprising a band of frequencies and a pair of spaced control frequencies located adjacent the limiting frequencies of the band, amplifying the energy, deriving from energy of one of the control frequencies a direct current voltage, varying the amplitude of said band frequencies by equal increments prior to amplification with said voltage, deriving from energy of the other control frequency a second direct current voltage, and varying the amplitude of said band frequencies prior to amplification with the differential of said two voltages.

HANS MAYER. 

